Method and apparatus for extending zero-voltage swiitching range in a DC to DC converter

ABSTRACT

Apparatus for extending a zero voltage switching (ZVS) range during DC/DC power conversion. The apparatus comprises a DC/DC converter, operated in a quasi-resonant mode, comprising (i) a transformer, (ii) a primary switch, coupled to a primary winding of the transformer, for controlling current flow through the primary winding, and (iii) a varactor, coupled to the transformer, for accelerating a downswing in a voltage across the primary switch.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of U.S. provisional patent applicationSer. No. 61/070,799, entitled “Apparatus for Extending Zero-VoltageSwitching Range in a DC to DC Converter”, filed Mar. 26, 2008, which isherein incorporated in its entirety by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the present invention generally relate to powerconversion and, more particularly, to an apparatus for extending thezero-voltage switching (ZVS) range in a DC/DC converter.

2. Description of the Related Art

A common topology for DC/DC converters is to operate a flyback converterin a quasi-resonant mode, where the primary switch is activated at thevalley of the drain voltage (i.e., a minimum point in the drain-sourcevoltage). The quasi-resonant flyback is a variation of the hard switchedflyback, which utilizes the parasitic capacitance of the switch, or evenan added capacitance, to absorb leakage inductance energy resulting froma leakage inductance of the DC/DC converter transformer. In addition, byadequately choosing the activation time of the switch, it is possible tohave a zero-voltage switching (ZVS) activation characteristic, as wellas ZVS deactivation characteristic, in order to improve overallefficiency.

One issue with such an approach is that a true ZVS transition onlyoccurs in a limited input voltage range and cannot be achieved for alloperating conditions. For example, the secondary reflected voltage hasto be higher than the input voltage to have a ZVS activation. If suchconditions are not met, the energy stored in the capacitance around theprimary switch is wasted as the voltage across the primary switch isre-set when the switch turns on, leading to a significant loss ofefficiency.

Therefore, there is a need in the art for the ability to extend the ZVSrange in DC/DC converters.

SUMMARY OF THE INVENTION

Embodiments of the present invention generally relate to apparatus forextending a zero voltage switching (ZVS) range during DC/DC powerconversion. The apparatus comprises a DC/DC converter, operated in aquasi-resonant mode, comprising (i) a transformer, (ii) a primaryswitch, coupled to a primary winding of the transformer, for controllingcurrent flow through the primary winding, and (iii) a varactor, coupledto the transformer, for accelerating a downswing in a voltage across theprimary switch.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the presentinvention can be understood in detail, a more particular description ofthe invention, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate only atypical embodiment of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

FIG. 1 is a schematic diagram of a DC/DC converter in accordance withone or more embodiments of the present invention; and

FIG. 2 is a graphical diagram of a drain-source voltage V_(ds) across aprimary switch in accordance with one or more embodiments of the presentinvention;

FIG. 3 is a schematic diagram of a DC/DC converter in accordance withone or more embodiments of the present invention; and

FIG. 4 is a flow diagram of a method for increasing a zero voltageswitching (ZVS) range in accordance with one or more embodiments of thepresent invention.

DETAILED DESCRIPTION

FIG. 1 is a schematic diagram of a DC/DC converter 120 in accordancewith one or more embodiments of the present invention. In someembodiments, the DC/DC converter 120 may be a flyback converter operatedin a quasi-resonant mode; alternative embodiments may comprise differenttypes of DC/DC converters, such as a buck converter, a boost converter,a buck-boost converter, and the like, operated in a quasi-resonant mode.The DC/DC converter 120 may be employed in a stand-alone configurationfor DC/DC power conversion, or may be utilized with or as a component ofother power conversion devices, such as a DC/AC inverter 124 as shown inFIG. 1. The DC/AC inverter 124 additionally comprises a DC/AC conversionmodule 122, coupled to the DC/DC converter 120, for converting an outputvoltage from the DC/DC converter 120 to an AC output voltage. The DC/ACinverter 124 may be utilized in the conversion of DC power, generated byone or more distributed generators (DGs) such as solar power systems, toAC power.

The DC/DC converter 120 comprises a capacitor 102 coupled across twoinput terminals of the DC/DC converter 120 for receiving an inputvoltage, V_(in). The capacitor 102 is further coupled across a seriescombination of a primary winding of a transformer 110 and asemiconductor switch 106 (“primary switch”). The primary switch 106 maycomprise one or more switches known in the art, such asmetal-oxide-semiconductor field-effect transistors (MOSFETs), bipolarjunction transistors (BJTs), emitter switched bipolar transistors(ESBTs), and the like. In some embodiments, a capacitor 108 is coupledacross drain and source terminals of the primary switch 106;alternatively, the capacitor 108 is not physically coupled across theprimary switch 106 but represents parasitic capacitances present at thenode, for example capacitances of the semiconductor primary switch 106,printed circuit board (PCB) capacitances, stray capacitances, and thelike.

A secondary winding of the transformer 110, having a 1:n turns ratio, iscoupled across a series combination of a diode 114 and an outputcapacitor 116, with an anode terminal of the diode 114 being coupled toa first terminal of the secondary winding. Two output terminals of theDC/DC converter 120 are coupled across the output capacitor 116 forproviding an output voltage, V_(out).

In accordance with one or more embodiments of the present invention, avaractor 112 (i.e., a voltage variable capacitor) is coupled across thediode 114; alternatively, the varactor 112 may replace the diode 114. Insome embodiments, the varactor 112 has a junction capacitance C_(var) asfollows:

$\begin{matrix}{C_{var} < {C_{0}*\left( {1 - \frac{V_{var}}{V_{j}}} \right)^{M}}} & (1)\end{matrix}$

where V_(var) is the varactor voltage and C₀, V_(j), and M arecoefficients dependent upon the specific varactor employed. The varactor112 may be comprised of diodes, MOSFETS, BJTs, ceramic capacitors, andthe like.

The DC/DC converter 120 receives the input voltage V_(in) and convertsthe input voltage to the output voltage V_(out). During such conversion,a current I_(c) flows through the capacitor 102 and a current I_(p)(“primary current”) is supplied to the primary winding of thetransformer 110 in accordance with the timing (i.e., opening andclosing) of the primary switch 106, as driven by a control circuit 104coupled to a gate terminal of the primary switch 106. When the primaryswitch 106 is open, no current flows through the primary winding of thetransformer 110 (i.e., I_(p)=0) and the current I_(c) charges thecapacitor 102. When the primary switch 106 is closed, the capacitor 102discharges and the primary current I_(p) increases linearly through theprimary winding of the transformer 110. The primary current I_(p)additionally flows through a leakage inductance of the transformer 110that is effectively in series with the primary winding.

When the primary switch 106 opens, the flow of the primary current I_(p)through the primary switch 106 ceases and the leakage inductancereverses its voltage, causing a rapid rise of a drain-source voltageV_(ds) across the primary switch 106 until the threshold voltage of thediode 114 is reached and the diode 114 begins to conduct. As a result ofthe energy stored in the magnetic field of the transformer 110, acurrent I_(s)˜I_(P)/n (“secondary current”) is induced in the secondarywinding and linearly declines to zero. As the secondary current I_(s)reaches zero, a core reset period begins and the drain-source voltageV_(ds) begins to ring sinusoidally at a frequency of an LC resonantcircuit seen from the primary side comprising the inductance of theprimary winding, the capacitance of the capacitor 108, and the reflectedcapacitance from the varactor 112, where the ringing is damped by ohmiclosses.

At the time the secondary current I_(s) reaches zero, the capacitance ofthe varactor 112 is large (i.e., approximately C₀) due to a varactorvoltage V_(var) close to zero. As the drain-source voltage V_(ds) beginsto swing down, the varactor voltage V_(var) increases. The risingvaractor voltage V_(var) reduces the varactor capacitance C_(var),thereby increasing the frequency of the LC resonant circuit during thedownward swing of the drain-source voltage V_(ds) and thus acceleratingthe downward swing of the drain-source voltage V_(ds). The accelerateddrain-source voltage downswing extends the zero voltage switching (ZVS)range by creating a deeper valley in the drain-source voltage V_(ds) forthe ZVS switching to occur. Thus, the primary switch 106 can beactivated at a V_(ds) closer to zero than that which would be possiblewithout the effect of the varactor 112. In some embodiments, the ZVSrange may experience at least a 30% increase.

FIG. 2 is a graphical diagram of a drain-source voltage V_(ds) across aprimary switch 106 in accordance with one or more embodiments of thepresent invention. The primary switch 106 operates within the DC/DCconverter 120 as previously described with respect to FIG. 1. Prior toT₀, the primary switch 106 is closed and current flows through theprimary switch 106. At time T₀, the primary switch 106 opens (i.e.,turns off), thereby terminating the flow of current through the primaryswitch 106. Additionally, the leakage inductance of the transformer 110reverses its voltage, causing a rapid rise of the drain-source voltageV_(ds). Once the threshold voltage of the diode 114 is reached, thediode 114 begins to conduct and a secondary current I_(s)˜I_(P)/n isinduced in the secondary winding and linearly declines to zero.

At time T₁, the secondary current I_(s) reaches zero and the varactorvoltage V_(var) is close to zero, resulting in a large capacitance ofthe varactor 112 (i.e., approximately C₀). A core reset period begins,and the drain-source voltage V_(ds) begins to ring at the frequency ofthe LC resonant circuit.

From time T₁ to T₂, as the drain-source voltage V_(ds) begins todecline, the varactor voltage V_(var) rises and reduces the varactorcapacitance C_(var), thereby increasing the resonant frequency of the LCresonant circuit during the downward swing of the drain-source voltageV_(ds). From time T₂ to T₃, the increased resonant frequency acceleratesthe downward swing of V_(ds), resulting in a V_(ds) downswing 202 thatis more rapid than a V_(ds) downswing 204 that would occur when theresonant frequency of the LC circuit remains unchanged (i.e., in theabsence of the varactor 112).

The accelerated V_(ds) downswing 202 results in a lower valley in thedrain-source voltage V_(ds) at time T₃ than a valley which would occurin the absence of the varactor 112, thus creating an extended ZVS range206. The extended ZVS range 206 allows the primary switch 106 to beactivated at a lower drain-source voltage V_(ds) (i.e., V₁) than thatwhich would be possible without the effect of the varactor 112 (i.e.,V₂), resulting in an energy savings of ½C*(V₂−V₁)², where C is thecapacitance of the capacitor 108.

FIG. 3 is a schematic diagram of a DC/DC converter 120 in accordancewith one or more embodiments of the present invention. In someembodiments, the DC/DC converter 120 may be a flyback converter operatedin a quasi-resonant mode; alternatively, the DC/DC converter 120 may bea buck converter, a boost converter, a buck-boost converter, or similartype of DC/DC converter. The DC/DC converter 120 may be employed in astand-alone configuration for DC/DC power conversion, or may be utilizedwith or as a component of other power conversion devices, such as theDC/AC inverter 124 as shown in FIG. 3. Additionally, as previouslydescribed, the DC/AC inverter 124 comprises a DC/AC conversion module122, coupled to the DC/DC converter 120, for converting an outputvoltage from the DC/DC converter 120 to an AC output voltage. The DC/ACinverter 124 may be utilized in the conversion of DC power, generated byone or more distributed generators (DGs) such as solar power systems, toAC power.

The DC/DC converter 120 comprises a capacitor 302 coupled across twoinput terminals of the DC/DC converter 120 for receiving an inputvoltage, V_(in). The capacitor 302 is further coupled across a seriescombination of a primary winding of a transformer 310 and asemiconductor switch 306 (“primary switch”). The primary switch 306 maycomprise one or more switches known in the art, such asmetal-oxide-semiconductor field-effect transistors (MOSFETs), bipolarjunction transistors (BJTs), emitter switched bipolar transistors(ESBTs), and the like. A voltage clamp circuit 308, comprising a diode318, a varactor 320, a capacitor 322, and a resistor 324, is coupledacross the primary switch 306 for controlling a spike in thedrain-source voltage created by leakage inductance energy from thetransformer 310, as further described below. Additionally, a capacitor312 is shown coupled across the primary switch 306 to representparasitic capacitances present at the node, such as capacitances of thesemiconductor primary switch 306, PCB capacitances, stray capacitances,and the like.

An anode terminal of the diode 318 and a first terminal of the varactor320 are coupled to a drain terminal of the primary switch 306; a cathodeterminal of the diode 318 and a second terminal of the varactor 320 arecoupled to a first terminal of the capacitor 322 and a first terminal ofthe resistor 324. A second terminal of the capacitor 322 and a secondterminal of the resistor 324 are coupled to a source terminal of theprimary switch 306. In some embodiments, the varactor 320 has a junctioncapacitance C_(var) as follows:

$\begin{matrix}{C_{var} < {C_{0}*\left( {1 - \frac{V_{var}}{V_{j}}} \right)^{M}}} & (2)\end{matrix}$

where V_(var) is the varactor voltage and C₀, V_(j), and M arecoefficients dependent upon the specific varactor employed. The varactor320 may be comprised of diodes, MOSFETS, BJTs, ceramic capacitors, andthe like. In one or more alternative embodiments, the varactor 320 mayreplace the diode 318.

A secondary winding of the transformer 310, having a 1:n turns ratio, iscoupled across a series combination of a diode 314 and an outputcapacitor 316, with an anode terminal of the diode 314 being coupled toa first terminal of the secondary winding; in some embodiments, thetransformer ratio may be below one (i.e., a step-down transformer). Twooutput terminals of the DC/DC converter 120 are coupled across theoutput capacitor 316 for providing an output voltage, V_(out).

Analogous to the operation previously described, the DC/DC converter 120receives the input voltage V_(in) and converts the input voltage to theoutput voltage V_(out). During such conversion, a current I_(c) flowsthrough the capacitor 302 and a primary current I_(p) is supplied to theprimary winding of the transformer 310 in accordance with the timing(i.e., opening and closing) of the primary switch 306, as driven by acontrol circuit 304 coupled to a gate terminal of the primary switch306. When the primary switch 306 is open, no current flows through theprimary winding of the transformer 310 (i.e., I_(p)=0) and the currentI_(c) charges the capacitor 302. When the primary switch 306 is closed,the capacitor 302 discharges and the primary current I_(p) increaseslinearly through the primary winding of the transformer 310. The primarycurrent I_(p) additionally flows through a leakage inductance of thetransformer 310 that is effectively in series with the primary winding.

When the primary switch 306 opens, the flow of the primary current I_(p)through the primary switch 306 ceases and the leakage inductancereverses its voltage, causing a rapid rise of the drain-source voltageV_(ds) that results in a spike well over the reflected voltage ofV_(out)/n. The resistor 324, capacitor 322, and diode 318 act as an RCD(resistor/capacitor/diode) clamp to limit such a spike and preventdamage to the primary switch 306.

As the drain-source voltage V_(ds) increases following the opening ofthe primary switch 306, the voltage across the diode 314 increases untilthe threshold voltage is reached and the diode 314 begins to conduct. Asa result of the energy stored in the magnetic field of the transformer310, a secondary current I_(s)˜I_(P)/n is induced in the secondarywinding and linearly declines to zero. Analogous to the operationpreviously described with respect to FIG. 1, when the secondary currentI_(s) reaches zero the drain-source voltage V_(ds) begins ringingsinusoidally due to an LC resonant circuit seen on the primary sidecomprising a capacitive component from the varactor 320, where theringing is damped by ohmic losses. As the drain-source voltage V_(ds)falls, the varactor voltage V_(var) increases and reduces the varactorcapacitance C_(var). The decreasing varactor capacitance C_(var)increases the frequency of the LC resonant circuit during the downwardswing of the drain-source voltage V_(ds), resulting in an accelerateddownswing of the drain-source voltage V_(ds). Such an accelerateddownswing extends the ZVS range by creating a deeper valley for the ZVSswitching to occur. In some embodiments, the ZVS range may experience atleast a 30% increase.

FIG. 4 is a flow diagram of a method 400 for extending a zero voltageswitching (ZVS) range in accordance with one or more embodiments of thepresent invention. The method 400 begins at step 402 and proceeds tostep 404. At step 404, a DC/DC converter is operated in a quasi-resonantmode. The DC/DC converter comprises a transformer having a 1:n turnsratio and may be a flyback converter, a buck converter, a boostconverter, a buck-boost converter, or similar type of DC/DC converter.In some embodiments, the DC/DC converter may be utilized in astand-alone configuration for DC/DC power conversion; alternatively, theDC/DC converter may be utilized with or as a component of other powerconversion devices, such as a DC/AC inverter 124. Such a DC/AC invertermay be utilized in the conversion of DC power, generated by one or moredistributed generators (DGs) such as solar power systems, to AC power.

At step 406, a switch (“primary switch”) of the DC/DC converter, coupledin series with a primary winding of the transformer, is activated forgenerating a current (“primary current”) through the primary winding,and the primary current linearly increases. At step 408, the primaryswitch is deactivated and the primary current ceases. Due to a leakageinductance of the primary winding, a drain-source voltage across theprimary switch rapidly increases until a diode coupled to thetransformer secondary winding is activated and a current (“secondarycurrent”) is induced in the secondary winding. In some embodiments, aspike in the drain-source voltage during such a rapid increase islimited by a voltage clamp circuit coupled to the primary winding.

The secondary current linearly declines to zero. Once the secondarycurrent reaches zero, the drain-source voltage begins ringingsinusoidally due to an LC resonant circuit of the DC/DC converter, wherethe ringing is damped by ohmic losses. The method 400 proceeds to step410.

At step 410, the frequency of the LC resonant circuit is increasedduring the downward swing of the ringing drain-source voltage, forexample by decreasing a capacitance of the LC resonant circuit duringthis time. In some embodiments, a varactor having a junction capacitancethat decreases as the corresponding varactor voltage increases may beutilized to provide a capacitive component of the LC resonant circuit,where the varactor voltage is increased as the drain-source voltagedecreases. Such a varactor may be coupled to the secondary winding ofthe transformer; alternatively, the varactor may be part of the voltageclamp circuit coupled to the primary winding. The increased resonantfrequency accelerates the downward swing of the drain-source voltage,creating a deeper valley (i.e., an extended ZVS range) for switching tooccur.

At step 412, the primary switch is activated at a valley of thedrain-source voltage, and a primary current flows through the primarywinding as previously described. In some embodiments, the primary switchmay be activated at the first valley of the ringing drain-sourcevoltage; alternatively, the primary switch may be activated at asubsequent valley. The method 400 proceeds to step 414, where a decisionis made whether to continue operation of the DC/DC converter. If theresult of such decision is yes, the method 400 returns to step 408; ifthe result of such decision is no, the method 400 proceeds to step 416where it ends.

While the foregoing is directed to embodiments of the present invention,other and further embodiments of the invention may be devised withoutdeparting from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

1. Apparatus for extending a zero voltage switching (ZVS) range duringDC/DC power conversion, comprising: a DC/DC converter, operated in aquasi-resonant mode, comprising (i) a transformer, (ii) a primaryswitch, coupled to a primary winding of the transformer, for controllingcurrent flow through the primary winding, and (iii) a varactor, coupledto the transformer, for accelerating a downswing in a voltage across theprimary switch.
 2. The apparatus of claim 1, wherein a capacitance ofthe varactor is decreased as the voltage decreases.
 3. The apparatus ofclaim 1, wherein the varactor is coupled to a secondary winding of thetransformer.
 4. The apparatus of claim 1, further comprising a voltageclamp circuit coupled across the primary switch for limiting thevoltage, wherein the voltage clamp circuit comprises the varactor. 5.The apparatus of claim 1, wherein the DC/DC converter further comprisesa diode and an output capacitor, wherein (i) an anode terminal of thediode is coupled to a first terminal of a secondary winding of thetransformer and a first terminal of the varactor, (ii) a first terminalof the output capacitor is coupled to a cathode terminal of the diode, asecond terminal of the varactor, and a first output terminal of theDC/DC converter, and (iii) a second terminal of the output capacitor iscoupled to a second terminal of the secondary winding and a secondoutput terminal of the DC/DC converter, wherein the first and the secondoutput terminals of the DC/DC converter provide an output voltage. 6.The apparatus of claim 1, wherein the DC/DC converter further comprisesan output capacitor, wherein (i) a first terminal of a secondary windingof the transformer is coupled to a first terminal of the varactor, (ii)a first terminal of the output capacitor is coupled to a second terminalof the varactor and a first output terminal of the DC/DC converter, and(iii) a second terminal of the output capacitor is coupled to a secondterminal of the secondary winding and a second output terminal of theDC/DC converter, wherein the first and the second output terminals ofthe DC/DC converter provide an output voltage.
 7. The apparatus of claim1, wherein the DC/DC converter further comprises a diode, a capacitor,and a resistor, wherein (i) an anode terminal of the diode is coupled toa first terminal of the varactor and a drain terminal of the primaryswitch, (ii) a cathode terminal of the diode is coupled to a secondterminal of the varactor, a first terminal of the capacitor, and a firstterminal of the resistor, and (iii) a second terminal of the resistor iscoupled to a second terminal of the capacitor and a source terminal ofthe primary switch.
 8. The apparatus of claim 1, wherein the DC/DCconverter further comprises a capacitor and a resistor, wherein (i) afirst terminal of the varactor is coupled to a drain terminal of theprimary switch, (ii) a second terminal of the varactor is coupled to afirst terminal of the capacitor and a first terminal of the resistor,and (iii) a second terminal of the resistor is coupled to a secondterminal of the capacitor and a source terminal of the primary switch.9. An inverter for extending a zero voltage switching (ZVS) range duringDC/AC power conversion, comprising: a DC/DC converter for converting DCinput power to DC output power, the DC/DC converter operated in aquasi-resonant mode and comprising (i) a transformer, (ii) a primaryswitch, coupled to a primary winding of the transformer, for controllingcurrent flow through the primary winding, and (iii) a varactor, coupledto the transformer, for accelerating a downswing in a voltage across theprimary switch; and a DC/AC conversion module for converting the DCoutput power to AC output power.
 10. The inverter of claim 9, whereinthe varactor is coupled to a secondary winding of the transformer. 11.The inverter of claim 9, further comprising a voltage clamp circuitcoupled across the primary switch for limiting the voltage, wherein thevoltage clamp circuit comprises the varactor.
 12. The inverter of claim9, wherein the DC/DC converter further comprises a diode and an outputcapacitor, wherein (i) an anode terminal of the diode is coupled to afirst terminal of a secondary winding of the transformer and a firstterminal of the varactor, (ii) a first terminal of the output capacitoris coupled to a cathode terminal of the diode, a second terminal of thevaractor, and a first output terminal of the DC/DC converter, and (iii)a second terminal of the output capacitor is coupled to a secondterminal of the secondary winding and a second output terminal of theDC/DC converter, wherein the first and the second output terminals ofthe DC/DC converter provide an output voltage.
 13. The inverter of claim9, wherein the DC/DC converter further comprises an output capacitor,wherein (i) a first terminal of a secondary winding of the transformeris coupled to a first terminal of the varactor, (ii) a first terminal ofthe output capacitor is coupled to a second terminal of the varactor anda first output terminal of the DC/DC converter, and (iii) a secondterminal of the output capacitor is coupled to a second terminal of thesecondary winding and a second output terminal of the DC/DC converter,wherein the first and the second output terminals of the DC/DC converterprovide an output voltage.
 14. The inverter of claim 9, wherein theDC/DC converter further comprises a diode, a capacitor, and a resistor,wherein (i) an anode terminal of the diode is coupled to a firstterminal of the varactor and a drain terminal of the primary switch,(ii) a cathode terminal of the diode is coupled to a second terminal ofthe varactor, a first terminal of the capacitor, and a first terminal ofthe resistor, and (iii) a second terminal of the resistor is coupled toa second terminal of the capacitor and a source terminal of the primaryswitch.
 15. The inverter of claim 9, wherein the DC/DC converter furthercomprises a capacitor and a resistor, wherein (i) a first terminal ofthe varactor is coupled to a drain terminal of the primary switch, (ii)a second terminal of the varactor is coupled to a first terminal of thecapacitor and a first terminal of the resistor, and (iii) a secondterminal of the resistor is coupled to a second terminal of thecapacitor and a source terminal of the primary switch.
 16. A method forextending a zero voltage switching (ZVS) range during DC/DC powerconversion, comprising: deactivating a primary switch of a DC/DCconverter operating in quasi-resonant mode, the primary switch forcontrolling current flow through a primary winding of the DC/DCconverter; and increasing a resonant frequency of a resonant circuit ofthe DC/DC converter during a downswing in a voltage across the primaryswitch to accelerate the downswing.
 17. The method of claim 16, whereinthe increasing a resonant frequency is caused by decreasing acapacitance of the DC/DC converter.
 18. The method of claim 17, whereinthe capacitance is altered by a varactor.
 19. The method of claim 18,wherein the varactor is coupled to a secondary winding of thetransformer.
 20. The method of claim 18, further comprising limiting aspike in the voltage, wherein the limiting is performed by a voltageclamp circuit coupled across the primary switch, wherein the voltageclamp circuit comprises the varactor.